Partridge Rock Roo + Black Sex Link Hen

Hello! We hatched a chick out of a Partridge Rock Roo + Black Sex Link Hen, the little chick is pure black with a little white-ish spot on its chest, would that mean it got the black sexlink gene from the hen & should be a pullet?

there's not a "sex linked gene", per se. With black sex links, it's based on the barring genetics. Males carry two copies and give one copy to each offspring, regardless of gender. Females carry one copy, and only give it to their sons. A black sex link is made when you put a non-barred rooster over a barred hen. The rooster has no barring genes to give, and the hen only gives them to the male. So, the male offspring gets one copy, which shows with the white spot on the head at hatch. The female chick gets none, so has the solid head.

But then, that female black sex link chick has no barring genes to potentially give to male offspring, so she herself can not make a sex-linked chick. You'll just have to wait for comb, etc like any other chick.